The invention relates to an EUV lithography apparatus and to a method for detecting particles present in the gas phase in an EUV lithography apparatus.
Although EUV lithography apparatuses are typically operated under vacuum conditions at (total) pressures of between approximately 10−9 mbar and approximately 10−1 mbar, contaminating substances, in particular contaminating particles, can pass into the interior of the EUV lithography apparatus (for example into the beam shaping system, the illumination system or the projection system) or can be formed there. If a large number of such particles present in the gas phase deposit on optical surfaces of optical elements present in the EUV lithography apparatus, this has a disadvantageous effect on the operation of the EUV lithography apparatus.
There are various causes or sources of contaminating substances or contaminating particles: it is known, for example, that light sources for EUV lithography apparatuses can be designed as plasma light sources. In the case of EUV plasma light sources (LPP, “Laser Produced Plasma”), droplets of molten tin are impinged upon by a pulsed carbon-dioxide high-power laser and evaporated in order to generate EUV radiation having a maximum intensity in the EUV spectral range between approximately 5 nm and approximately 20 nm. In the case of such light sources, tin particles from the vicinity of the light source can pass into the beam shaping system, in which the light source is situated, and from there via the illumination system right into the projection system.
In EUV lithography apparatuses, reflective elements, in particular mirrors, are typically used as optical elements since no optical materials having a sufficiently high transmission exist at the wavelengths used there. Such optical elements can be arranged in a mechanically pivotable manner in order to be able to influence the beam path in the interior of the EUV lithography apparatus. By way of example, so-called facet mirrors can be provided in the illumination system, said facet mirrors having a plurality of tiltable individual mirrors in order to realize different illumination settings. In the course of the tilting of the individual mirrors, mechanical abrasion in the form of particles may possibly be established, said particles merging with the residual gas atmosphere. Also, the substrate (“wafer”) to be patterned or a light-sensitive layer (“resist”) provided thereon or components incorporated in the EUV lithography apparatus can outgas contaminating substances or particles which can deposit on optical surfaces and can have a disadvantageous effect on the operation thereof in the EUV lithography apparatus.
The contaminating substances or particles can be, inter alia, heavy hydrocarbons which, under irradiation with EUV radiation, deposit as solid carbon on the mirror surfaces, as a result of which the reflectivity of the mirrors decreases in a detrimental manner. Such deposits may possibly not be completely removed from the optical surfaces even in the event of cleaning, with the result that the lifetime of the optical elements decreases. Contamination by the deposition of particles can affect not only mirror elements but also the (reflective) mask whose structure is intended to be transferred to the substrate to be patterned, and also the wafer or the light-sensitive substrate.